Similar to what I recommended to Mamalala, I would say that you should just throw things out there without any expectation of him reciprocating. Not questions... Just little hot requests or statements on what you are liking at the moment. Rather than have a discussion about it, which will make him nervous because he probably has no idea what to do, SHOW him what you mean.
He may not reciprocate right away, but I think he eventually will because he will enjoy what you're saying and because he'll catch on that it's not really as scary as it seems.
I think talking dirty, for some, can be similar to public speaking... terrifying. What if I sound stupid? What if I say the wrong thing? What if she doesn't like me? So, I would diffuse that fear by *giving* for a little while until he is more comfortable with the idea.
